If a client seeks me (or an AI tool) out because they think they need "just some simple proofreading," this means that they do not trust themselves to catch proofreading-style mistakes. And I am making the point that, if you are already so weak at writing that you know need outside help to find even your simple mistakes, your writing probably has big problems too — and you likely don't realize how much deeper those problems go. (Or at least this is the case for native-speaker writers. ESL writers are inclined to make simple mistakes just because of the language gap, without it meaning that their fundamental writing skills are lacking.)

I am definitely not calling these people arrogant, careless, etc. I respect my clients and don't assume anything negative about their characters based on their writing skills (...or writing challenges). So what I mean here is indeed just about self-trust. ;)

But yes, you also raise a useful example: AI would fail, most likely, to catch actual mistakes in word choice, at least in more subtle situations. This is also a huge part of my job. The social/emotional intelligence component: "This word means [X]. I think you are using it to mean [Y] instead, but it doesn't work that way. Is that so? If yes, then the following alternative words might work better...."